1. Field of the Invention
The present invention relates generally to a method for forming a thermoplastic tubular jacket and, more specifically, to a method of manufacturing a polyethylene jacket useful as an outer protective jacket for an insulated pipe.
2. Description of the Prior Art
There are many instances in which insulated pipelines are needed. For example, distributed HVAC (heating, ventilation and air conditioning) applications utilize chilled water for cooling and steam for heating. The chiller and boiler are typically contained in a central location and the chilled water and steam are distributed to other locations. For example, on a school campus, the chiller and boiler may be located in a power plant building. The chilled water and steam are distributed to classrooms in separate buildings.
A set of insulated pipelines is used to convey the chilled water from the chiller to other locations and back to the chiller. Another set of insulated pipelines is used to carry the steam from the boiler to the other locations and back to the boiler. The insulated pipelines are usually located underground.
Insulated pipe is conventional and commercially available. One type of insulated pipe uses a steel pipe to convey fluid. Around the outside of the steel pipe is a layer of insulating foam such as, for example, polyurethane foam. Around the outside of the foam is a jacket of hard thermoplastic (such as high density polyethylene, HDPE). The plastic jacket protects the foam from mechanical damage and also provides a water tight seal to prevent corrosion of the steel pipe.
HDPE jackets are commercially available from a number of sources. However, a need exists for a simpler and more economical method for producing such jackets.
The present invention has as its object to provide a simpler and more economical method for producing thermoplastic tubular jackets. The present method is simple to carry out and produces, for example, HDPE tubular jackets at reduced cost for incorporation into pre-insulated piping.
In the method of forming a thermoplastic tubular member of the invention, a mandrel is provided having an exposed surface area. The mandrel is coated with a destructible coating which substantially covers the exposed surface area. A hot melt profile of thermoplastic material is then extruded onto the previously applied destructible coating and is spirally wound along the mandrel to produce adjacent windings. The adjacent windings of the melt profile are arranged to make contact to form a continuous tubular member on the mandrel which surrounds the destructible coating. After the melt profile has cooled, an external force is applied to the tubular member sufficient to destroy the integrity of the destructible coating whereby the tubular member so formed can be extracted from the mandrel.
Preferably, a rotatable mandrel is used in the method which is coated with a friable coating. The friable coating can be, for example, a high temperature polyurethane foam. After the melt profile has been extruded and spirally wound over the friable coating, an external compressive force is applied to the tubular member in order to crush the friable coating. The external compressive force can be applied, for example, by series of rollers located on a production line downstream from the point at which the melt profile is extruded.
Additional objects, features and advantages will be apparent in the written description which follows.